Big earthquakes can trigger temblors across globe, USGS says

USGS The incidence of magnitude 5.5-or-greater earthquakes increased across the globe after a magnitude 8.6 temblor an April.

A large earthquake in one part of the globe can trigger earthquakes elsewhere, according to new research by scientists from the U.S. Geological Survey and UC Berkeley.

The global aftershocks are fairly immediate, taking place within a week of the original large quake, the researchers said. And the observation might require seismologists to change their definition of an aftershock, from one that stresses quakes caused in the immediate region of an earthquake to one that can occur anywhere.

“Earthquakes are immense forces of nature, involving complex rock physics and failure mechanisms occurring over time and space scales that cannot be re-created in a laboratory environment,” USGS Director Marcia McNutt said in a press statement.

“A large, unusual event such as the East Indian earthquake last April is a once-in-a-century opportunity to uncover first order responses of the planet to sudden changes in state of stress that bring us a little closer to understanding the mystery of earthquake generation,” she said.

In April, a magnitude 8.6 earthquake struck the East Indian Ocean along a strike-slip fault – the largest earthquake ever recorded on a strike slip and 10 times larger than any previously recorded strike-slip quake.

"If you asked any of us if this event is possible a year ago, we would have laughed at you," said Thomas Heaton, a seismologist at the California Institute of Technology, who was not involved with the current study. "Because this was such an unusual event, it radiated very large seismic waves globally."

In the six days that followed the April quake, the number of earthquakes across the globe that were magnitude 5.5 or larger increased nearly fivefold.

“This was a really surprising finding,” said Fred Pollitz, a seismologist with the USGS and lead author of the paper. “We’d been telling people for years that this sort of thing couldn’t happen.”

Other quakes that have equaled or exceeded magnitude 8.5 have increased the number of magnitude 5.5-or-greater quakes globally, but only at one-third the rate as this year's quake.

Pollitz speculates that quakes along strike-slip faults might cause more of a ruckus globally than ones along thrust faults because strike slips radiate their energy horizontally, while the energy from a thrust fault is more vertical.

Also, the April quake was of very short duration for a large quake, said Pollitz, who said it lasted between 90 seconds and two minutes. The energy was therefore released quickly. In contrast, the magnitude 9.0 Tohoku quake that hit the coast of Japan in March 2011 lasted closer to six minutes.

“It was a freak thing,” he said of the East Indian Ocean quake, but an amazing opportunity for seismologists to observe a global event.

He said that the timing of these aftershocks is short, in the range of a few days, not years. Therefore, it is unlikely the large earthquakes in the past few years – the magnitude 8.6 in the East Indian Ocean, the 2011 Japanese quake, the 2004 Sumatran quake – are related.

“We can’t think of any mechanism that would explain that,” he said.

Heaton said that while the research is interesting, it is unlikely to aid disaster awareness in earthquake-prone regions.

"I think that it is unlikely that we will be able to use it to tell when to take cover," he said. "In the meantime, our primary defense against earthquakes is to construct resilient structures."